1. Field of the Invention
The invention relates to the field of local computer networks and finds particular application when used with a high speed digital serial bus such as coaxial cable.
2. Description of the Prior Art
Due to technological trends that are reducing the cost of computation (for example, memory and microprocessor costs), the proliferation of high performance workstations is forecast for the so-called office of the future. Such workstations shall be capable of both word and data processing applications. However, the cost of semiconductor devices has historically dropped approximately fifty percent every two years while the cost of mechanical peripheral devices (for example, disk drives, tape drives, and printers) has not so eroded. This fact has tended to make the sharing of such relatively expensive peripherals between workstations an economic necessity. While computer peripheral devices have been interconnected in the prior art using input/output channels and relatively low speed digital links, a general purpose high speed digital serial link that presents a standard interface to which multiple manufacturers of equipment can design is highly desirable. The specifications for such a high speed digital serial link have been proposed and are known generically as the Ethernet.
The original Ethernet technology was the subject of U.S. Pat. No. 4,063,220 to Metcalfe et al. (filed Mar. 31, 1975; issued Dec. 13, 1977). Metcalfe et al. describes a multipoint data communication system with collision detection. A communication medium is utilized to permit the high speed serial transmission of packets of information which are transmitted and received by transceivers connected to said communication medium. Said medium of a preferred embodiment was disclosed to be coaxial cable. Further, a practical implementation of a transceiver for use with the multipoint data communication system with collision detection is disclosed in FIG. 5. However, the disclosed transceiver is subject to many technical and/or performance limitations:
The transceiver as disclosed utilizes DC coupling at the receiver input necessitating strict control over (1) the absolute DC level on the coaxial cable and (2) the receiving DC reference threshold.
Signalling hysteresis must be maintained at extremely small allowable levels lest signal symmetry degrade with long coaxial cable lengths.
The details for the practical supply of voltages V+ and F+ are not disclosed, but severely impact the practical implementation of such a device.
The methods of collision detection would have to be changed from the disclosed design for operation at high signalling rates (for example, 10 megabits per second) and for operation with AC coupled receiver input.
Single-ended signalling is utilized in the interface cable leads.
Extreme care must be exercised in the design of the isolation stages as disclosed lest undesirable radio frequency interference be generated.
The transceiver as disclosed is unable to assist in the location of coaxial cable impedance discontinuities or faults.
The transceiver as disclosed is also subject to failure caused by voltages on the coaxial cable opposite in polarity from the normal signalling polarity.
Further information on the Ethernet has been also disclosed in four technical reports:
Ethernet: Distributed Packet Switching for Local Computer Networks, by Robert M. Metcalfe and David R. Boggs, originally published by the Xerox Palo Alto Research Center, May, 1975.
Practical Considerations in Ethernet Local Network Design, by Ronald C. Crane and Edward A. Taft, originally published by the Xerox Systems Development Division and Palo Alto Research Center, October, 1979.
Measured Performance of an Ethernet Local Network, by John F. Shoch and Jon A. Hupp, originally published by the Xerox Palo Alto Research Center, February, 1980.
Evolution of the Ethernet Local Computer Network, by John F. Shoch, David D. Redell, Yogen K. Dalal, and Ronald C. Crane, published by the Xerox Palo Alto Research Center and the Systems Development Department, February, 1981.
While the above technical references are relevant to an understanding of the physical signalling characteristics on an Ethernet coaxial cable and link level control procedures, they do not disclose sufficient information required by one skilled in the art to construct a high speed Ethernet transceiver.
In addition to the above references, an Ethernet specification proposing a 10 megabit per second standard coaxial cable based serial bus was published on Sept. 30, 1980, jointly by Digital Equipment Corporation, Intel Corporation, and Xerox Corporation (version 1.0). The proposed 10 megabit per second Ethernet joint specification is insufficient with respect to details as to how to implement a transceiver to function at such signalling frequencies. Further, the performance limitations of the transceiver as disclosed by Metcalfe et al. become exacerbated at 10 megabits per second, the actual design being intended to operate at slightly less than 3 megabits per second (reference the May, 1975, paper by Metcalfe and Boggs).